1. Field of the Invention
The invention relates to broadband interferometry and, more particularly, to an interferometry system and method in which all components of a complex interferometric signal can be simultaneously acquired.
2. Background of the Related Art
Interferometry with broadband light sources has become a widely used technique for imaging in biologic samples using time-domain optical coherence tomography (OCT), optical coherence microscopy (OCM), spectral domain OCT (which encompasses spectrometer based Fourier domain OCT and swept source OCT), color Doppler OCT, and phase-referenced interferometry. In all of these interferometry techniques, light traveling a reference path is mixed with light returning from or traversing a sample on the surface of a single or multiple detectors.
In homodyne interferometry, the optical frequency of the sample and reference light is the same, and mixing of the fields on the detector results in sum and difference frequency terms corresponding to a second harmonic frequency component and a DC frequency component. The second harmonic frequency component is at twice the optical frequency, and is therefore not resolved by conventional square-law electronic detectors.
In heterodyne interferometry, either the reference or sample arm light is purposefully modulated at a carrier frequency, which results in the difference frequency component residing on a carrier frequency which is electronically detectable. The complete interferometric signal consists of DC components arising from non-mixing light from each of the arms, and interferometric components arising from mixed light. In heterodyne interferometry it is straightforward to separate the DC from interferometric components, since the latter are distinguished by their carrier frequency. In homodyne interferometry, it is impossible to separate the interferometric and non-interferometric components based on their frequency content alone.
In both homodyne and heterodyne interferometers, the interferometric component of the detector signal depends sinusoidally on both the optical path length difference between the arms of the interferometer, and also on an additional phase term which specifies the phase delay between the reference and sample arm fields when the path length difference is zero. When this phase term is zero, the interferometric signal varies as a cosine of the optical path length difference between the arms, and when the phase term is 90 degrees, the interferometric signal varies as a sine of the path length difference. Although a single detector can only detect one of these phase components at a time, it is convenient to refer to the zero and 90 degree phase delayed versions of the interferometric signal as the real and imaginary components (or zero and 90 degree quadrature components) of a complex interferometric signal.
In OCT and many of its variations discussed above, it is often useful or necessary to have access to the entire complex interferometric signal in order to extract amplitude and phase information encoding, scatterer locations and/or motions. For example, in Doppler OCT, multiple phase measurements are required to extract the magnitude and direction of sample motions. In spectral domain OCT, acquiring only one quadrature component of the interferometric signal results in a complex phase ambiguity which does not allow for separation of image information resulting from positive and negative spatial frequencies of the detected data. This ambiguity results in double images, image contamination with undesirable autocorrelation terms, and is wasteful of detector pixels (in Fourier domain OCT) and data collection time (in swept source OCT). Unfortunately, the square-law detector output which is available in previously disclosed OCT systems and their variations obtains only the real part of the complex signal (in the case of single receiver systems), or the real part and its inverse (in the case of differential receiver systems).
Several methods have been reported which allow for instantaneous or sequential retrieval of both quadrature components of the complex interferometric signal. These include 1) polarization quadrature encoding, where orthogonal polarization states encode the real and imaginary components; 2) phase stepping, where the reference reflector is serially displaced, thus time encoding the real and imaginary components; and 3) synchronous detection, where the photodetector output is mixed with an electronic local oscillator at the heterodyne frequency. Synchronous detection methods include lock-in detection and phase-locked loops. Polarization quadrature encoding and phase stepping can be generically called quadrature interferometry since the complex signal is optically generated. As such, they are useful in both homodyne and heterodyne systems.
Each of these techniques suffers from shortcomings. Polarization quadrature encoding is instantaneous, but it requires a complicated setup, and suffers from polarization fading. Phase shifting requires a stable and carefully calibrated reference arm step, is not instantaneous, and is sensitive to interferometer drift between phase-shifted acquisitions. Synchronous detection is not instantaneous, and depends on the presence of an electronic carrier frequency. Systems based on synchronous detection are thus not useful in an important class of homodyne systems, such as en-face imaging schemes, those which take advantage of array detection (e.g. Fourier domain OCT), and in swept source OCT.
There is thus a clear need for a system and method for instantaneous and simultaneous acquisition of both quadrature components of the complex interferometric signal in OCT and related systems.